According to Brown, before the Flood, the Earth's crust floated on a thick layer of water, above the mantle. Walls and tendrils connected the mantle and crust, allowing the inner and outer reaches of the planet to rotate on its axis at the same speed.

But the fantasy does not stop there; according to this "theory" antediluvian Earth had one super-continent—similar in concept to, but not the same as, Pangaea—that covered about 75 percent of the surface. Oceans, if you could call them that, were really giant lakes (like the Sea of Galilee, the Red Sea, etc.). To maintain consistency they are obliged to argue that Earth's mountains rarely reached more than 5000 feet (1524 meters) above what was then sea level and the highest mountain was probably much lower than 9000 feet (2743 meters).

When it came time to flood Earth, God cracked the crust, releasing the water (like a soft-boiled egg). These cracks formed underwater mountain ranges like the Mid-Atlantic Ridge. The subterranean water sprayed upward with such great force that it caused worldwide rainfall for 40 days and 40 nights (and made the comets, asteroids, Trans-Neptunian Objects, and anything else they don't understand). The water left its subterranean encasement so quickly that many land masses immediately began to sink. These rapidly sinking land masses uplifted other land masses, thus causing deep-sea fossils to appear at the tops of mountain peaks. (Creationists always point to "mountaintop fossils" as proof of the flood — naturally wanting to wish away the vast amount of evidence for slow-motion tectonic forces taking millions of years to raise mountains from the ocean floor.) In order to explain away radiometric dating, Brown also claims that all radioactive elements were created during the Flood, and subsequently decayed at rates billions of times greater than today, explaining why radiometric dating appears to indicate a billions-of-years-old Earth.

The proponents of this pseudoscience claim that many of the sunken land masses remain sunken today, forming the world's ocean floors. All of these sudden movements caused the super-continent to develop tectonic ridges and mountain ranges that run parallel to Earth's coastlines. These movements also created, in the eyes of creationists, an illusion that led scientists to develop the theory of Pangaea.

The ocean floors took less than six months to sink to their current depths. The flood waters then filled these troughs, creating the oceans and drying out the deluged land.

This idea is also being prepared as a pre-emptive strike against any living microbes that might be found on Mars or other parts of our Solar System. Creationists are claiming that some mud might have gone flying out into space, and landed on another planet.

Walter Brown, as a mechanical engineer, goes into some detail about the math around his theory. Let's take some time to see what effects these would have on the Earth. (Spoiler: it destroys the Earth)

Brown says the water-jets from his subterranean ocean created "comets, asteroids, meteoroids, and TNOs," and were propelled from the earth at velocities at least 32 miles per second (51.5 km/s) (which is what's required to create long-period retrograde comets). We have data on how much stuff this would require. We know of at least around 2,000 comets[1] and comets are usually around 5×1014 kg[2] for a total cometary mass of around 1×1018 kg. Brown's table of comets shows that around 18% of all comets are retrograde. The mass of the asteroid belt is around 3.0×1021 kg.[3] While there's no solid data on the mass of all Trans-Neptunian Objects (TNOs), the mass of a subset of TNOs called the Kuiper Belt is around 0.02 Earths,[4] or about 1.2×1023 kg,[5] and no more than 0.1 Earths. Adding them all together is about 1.23×1023 kg, with 97.6% of that coming from the TNOs.

The most efficient way of getting an object from one orbit to another is called a Hohmann Transfer Orbit, and we can calculate how much energy (measured as delta-V, or the velocity change required) it takes to enter such an orbit. To get from the orbit of Earth to the orbit of Pluto takes about 11.8 km/s, or 16.1 km/s if you add in the energy required to escape Earth's gravity.[6] Note that's the minimum energy required to get something to the distance of Pluto, if it's propelled at precisely the right angle from Earth (prograde).[note 1] Similarly, the energy required to put the asteroids into orbit would be between 11.3 km/s and 14.0 km/s, depending on how far up the belt the asteroid needs to go. (This ignores the few km/s delta-V required to later put those objects into circular orbits once at their destination altitude.)

Using these values, we can calculate the energy required to get these objects to their current altitudes (again, putting them into their orbits would take a bit more energy):

As with the mass, the vast majority of energy comes from the TNOs. Mind this is just the minimum energies required to put what we see today to the altitudes at which we see them, if everything was shot at just the right angles to do this. However, the Earth rotates,[citation needed] which compounds the problem. These fountains lasted weeks, after all. Because of the rotation of the Earth, jets would only spend two hours of every day within 15 degrees of prograde (since the vast majority of the energy came from ejecting TNOs prograde, we can ignore the relatively tiny bits of retrograde material), and half of every day pointing the other way. That means the amount of mass that left Earth would have to be at least double the current TNO mass, since at most half of it would eventually become the TNOs.

But we can do better than just say "at most half", since the amount of velocity prograde to Earth's orbit is a simple trigonometric function of where the fountain is in relation to prograde. If we assume that the average velocity of material from Earth was just the minimum 16.1 km/s, then only the material very close to (say less than a degree away from) prograde would become TNOs, which means we'd need to multiply the mass ejected from Earth by 360, because only 1/360th of the ejecta would be traveling along the right vector to form TNOs.

If the velocity increased, the range of angles would increase as well, since things slightly offset from the prograde would still have enough energy to reach TNO orbits. This poses another problem, because anything leaving the Earth faster than 16.5 km/s will leave the solar system entirely. This leaves a very small range of velocities in which something from Earth will become a TNO. To maximize the angles we need an exit velocity of exactly 16.5 km/s,[7] which gives a window of 12.6 degrees on either side of prograde in which material would reach TNO altitudes. That would require multiplying the mass by 14.3, because only about 7% of material would become TNOs.[note 3] (Using Brown's maximum velocity of 51.5 km/s leaves only two 0.5 degree windows).

So now we can update our energy equation:

Class of Object

Mass

Velocity

Energy

TNOs

1.7×1024 kg

16.5 km/s

9.3×1032 J

How much energy is that? Well, the Chicxulub Impactor delivered 9,300 million times less energy than that.[8] Or, to put it another way, it's as if the Earth were hit by 2691 Chicxulub impacters per second for 40 days straight.[9]

But let's go bigger. The Earth's gravitational binding energy — the energy it would take to destroy the Earth Alderaan-style — is about a quarter of creating TNOs: 2.49×1032 J. The total energy the sun produces over a whole year is only 13 times bigger than what what ejecting the TNOs produced in 40 days.[note 4] — meaning that for those 40 days of fountaining, the Earth produced at least roughly the same amount of total energy as the Sun. The Earth is a lot smaller than the Sun,[citation needed] which means the energy would have to be more concentrated. The Sun's energy flux is about 6.33×107 W/m2,[10] while, during the fountaining, the Earth's would've been over 8,000 times higher at 5.3×1011W/m2.[11].

That energy has to go somewhere,[note 5] and since it didn't go into Earth's kinetic energy (Earth's rotation would more or less cancel out the acceleration produced by the jets) or rotational energy (that would mess with the meaning of "day", and that's treading too close to Day-age creationism, which obviates the need to creating celestial objects via the flood and therefore is unacceptable[note 6]), the only options are light, heat, or deformation (kinetic). It certainly would have deformed the Earth — it was, after all, surrounded by what are described by Brown as "rocket engines" providing a continuous force of 8.1×1021 Newtons[12], but we know no more than a quarter of that energy could go into deformation, because otherwise the Earth wouldn't exist anymore. That leaves heat and light to make up the remaining 7.1×1032 J (or 4×1011 W/m2). If even one millionth of that energy were converted to heat, it would've melted the surface of the Earth.[13] That means it'd have to be light, with a peak frequency in the far ultra-violet and around 6.7 million times brighter than the sun at zenith.[14]. Right at the wavelength liquid water is most opaque,[15], so the flood-waters would've absorbed most[note 7] of that energy. Even if it only absorbed one six-hundred-thousandth the light-energy,[note 8] that's more than enough to boil all the oceans.[16] No matter how you slice it, the energy just in creating the trans-neptunian objects is enough to boil the oceans, melt the crust, and fully obliterate the Earth.[note 9]

(Spoiler: No it doesn't.)
Recently, a discovery was made from a battered ringwoodite that survived a trip from the mantle[18], indicating a long-held theory that there could be a very, very large amount of water that's trapped in a really distinct layer in the deep Earth. Creationists, ever the straw-graspers, jumped on this discovery and started spamming it everywhere as "proof" of the validity of hydroplate theory.

However, it should be pointed out that those who post and point to this discovery as "proof" are committing a common error among creationists: Not actually reading the article beyond the title. If they had, they would find that first of all the notion that water could simply "explode" out from the mantle is quite frankly ridiculous. Secondly, unlike what the title alone suggests, scientists did not find some "ocean of liquid water just sitting in this transition layer waiting to be spilled out on a moment's notice", but rather incredibly high-pressure concentrations of non-liquid ringwoodite that contain "water" in the form of hydroxide ions.

Malachite

Expecting water to spring forth from ringwoodite is like expecting water to suddenly gush out of Malachite, a mineral that is used in the forging of Glass armor and weapons contains hydroxides at atmospheric pressures. If these hydroxide ions suddenly exploded out on the surface and were suddenly released by some magic they aren't going to just turn into water. Much of it would turn into caustic chemicals like lye at tremendously high temperatures. Noah's large wooden boat wouldn't be much protection against being steamed for 40 days in ~1000 °C lye[19].

What they found was not really an "ocean of water below the crust" but rather a ring of ringwoodite between the upper and lower mantles. Hardly anything that could have caused a global flood through plate tectonics or any other volcanic mechanisms. It's akin to how probes have found water on the Moon, but it's not liquid water — it's bare traces of it that could potentially be mined to extract small amounts of water for drinking, etc.

The only thing this discovery really "proves" for creationists is just how desperate they are to vindicate themselves. Of course, at this point it should be pretty obvious.

↑Brown's 51 km/s number is because some long-period comets orbit retrograde (against Earth's orbit), which requires negating Earth's own 29.8 km/s orbital velocity in addition to then putting the object into solar orbit.

↑Calculated from the total energy and total mass, not added or averaged from the velocities column

↑This assumes that the fountains only existed directly beneath the plane of the ecliptic, rather than everywhere on Earth. If only the fountains within 12.6 degrees of the ecliptic could contribute, and fountain ejecta mass were the same for all latitudes, and all latitudes could contribute to TNOs if they pointed within 12.6 degrees of prograde, then only about 0.5% of mass could become TNOs, requiring a multiplication value of 204

↑Brown only says the fountains were open for a few weeks, but the impression the Bible gives is that the fountains were closed around the same time the rains stopped, for a total duration of 40 days

↑Newton's Third Law says that every action has an equal and opposite reaction. Earth launching TNOs into space means the TNOs pushed back against the Earth with the same force.

↑Ejecting TNOs requires over 4,000 timesEarth's rotational energy. Even a tiny fraction of a percent of that going into rotational energy would've been enough to completely reverse Earth's spin, or start it up from near-tidally-locked with the Sun,
meaning a "day" pre-flood could've lasted a million years for all we know.

↑Or about 1.6×10-6, a level of transparency water doesn't have for any part of the EM spectrum. The most transparent water gets is around 500 nm, where it approaches only 5×10-2, 30,000 times more absorbent than 1.6×10-6

↑Keep in mind this is the best-case scenario. This uses the absolute minimum amount of mater to be ejected from only the latitudes that could get it to TNO altitudes, propelling it at the minimum speed required, while distributing the resulting force evenly across the entire surface of the Earth. The actual model has the force concentrated in narrow bands running around the Earth (including latitudes outside the band near the ecliptic, which would increase the total amount of mass ejected significantly). As xkcd showed, constraining ejecta flow like that increases the energy required, as well as localizing its impacts which would transfer even more of that energy into kinetic-deforming forces (which were already well over the amount required to do to Earth what the Death Star did to Alderaan).

↑Delsanti, Audrey & Jewitt, David. "The Solar System Beyond The Planets" (PDF). Institute for Astronomy, University of Hawaii. Archived from the original (PDF) on September 25, 2007. Retrieved March 9, 2007.

↑Heat capacity of liquid water is 4180 J/(LK), so to raise 1 L of water by 200 K would take 8.36×105 J. The photonic energy is at least 7.1×1032 J. Distributed over the 1.386×1021 Liters of water on Earth is 5.33×1011 J/L, 637,560 times more than enough to boil it all. Keep in mind that Earth's oceans are between the energy being absorbed by the Earth and the ice shooting into space, and thus before the fountains can absorb the light, the oceans must first boil away.